Zwitterionic Halido Cyclopentadienone Iron Complexes and Their Catalytic Performance in Hydrogenation Reactions

The zwitterionic halido cyclopentadienone iron complexes FeX(CO) (X = Cl, Br, I) were prepared and characterized by NMR, XRD, MS, IR, and elemental analysis. Their catalytic performance in hydrogenation and transfer hydrogenation was assessed. Transfer hydrogenation in boiling ⁱPrOH with acetophenone as the test substrate showed no conversion with FeI(CO). Hydrogenation reactions under H₂ pressure (7.5 bar) in water as solvent showed up to 93% conversion with FeI(CO) (2.5 mol %) using acetophenone as the test substrate. The overall relative reactivity order was established to be Cl < Br < I, reflecting the relative bond strengths of the Fe-X bonds. Although the compounds presented in this study can be used as precatalysts for hydrogenation reactions in water, the need to employ high temperatures, leading to more catalyst decomposition, as evidenced by pressurized sample infusion-electrospray ionization-mass spectrometry (PSI-ESI-MS), and high catalyst loading limits their usefulness as catalysts. The limit can be circumvented in part by salt effects analogous to those in classical solvolysis chemistry.